1. Field of the Invention
This invention relates to a structure that can regulate an air gap magnetic flux (density) in a radial gap type motor and easily regulate a magnetic force for acting in an axial direction between a rotor magnet and a stator core.
2. Description of the Prior Art
A radial gap type motor or particularly an outer rotor type increases a volume of a rotor magnet and increases an opposed area between a stator yoke (a salient pole of a stator) and the magnet, and is hence generally suitable for the utility of the type required for a low speed and a high torque. For instance, a spindle motor of a direct drive type floppy disk drive (hereinafter referred to as an "FDD") is mostly of this type.
However, the following problems arise since a magnet volume is large and the opposed area to the stator yoke is large.
(1) A cogging torque and a torque ripple are large, and when a speed is controlled, a rotation uniformity is deteriorated. Accordingly, it is necessary to regulate an air gap magnetic flux (density) distribution by some methods and to reduce the cogging torque.
(2) If a magnetic balance between the rotor magnet and the stator yoke in an axial direction is not provided (generally in a structure that an attraction force is applied from a bearing structure in an axial direction), a magnetic attraction force is always acted between the rotor magnet and the stator yoke, and this force is acted to a bearing. In this case, in the bearing structure where two ball bearings are used, the force is not so troublesome but there is a disadvantage in an increase in a bearing cost. On the other hand, in a sleeve bearing structure, a radial clearance of about several microns is required between the bearing and the rotational shaft, the axial deflection of the rotating rotor is increased (deteriorated) due to the magnetic unbalance between the rotor and the stator according to this clearance, and hence the rotating accuracy is decreased.